1. Field of the Invention
The present invention relates to a control system for a variable focus lens in general, and to such a control system for a variable focus lens in a photographic camera, in particular.
2. Description of the Prior Art
Variable focus lenses and apparatus utilizing such lenses such as photographic cameras, telescopes and the like are well known and their operation are well understood in the prior art. A camera having a variable focus lens and having a focus control system coupled to said lens for automatically focusing image-containing light rays of a remote object at the film plane of said camera in response to a rangefinder derived signal representative of the actual distance between said camera and said remote object, are known in the prior art. A camera having such a focus control system that utilizes acoustical energy to determine the distance to an object remote from said camera is described in U.S. Pat. No. 3,522,764 to BIBER et al.
A bidirectional focus control system, i.e., a focus control system that, once commanded to do so, can move the movable element of a variable focus lens in either of two directions from any initial position within the normal focusing range of said lens, for focusing purposes, is also known in the prior art. One example of such a system is described in the above-mentioned BIBER et al. patent.
Another type of focus control system for a variable focus lens is sometimes referred to as an unidirectional focus control system in that the movable or focusing element of such a lens is always moved from the same or known reference point for focusing purposes. While such an element is movable in either of two axial directions, no lens focusing is performed as said element moves within its focusing range towards said reference point. A unidirectional focus control system of this type is signficantly less complex than a bidirectional focus control system because it is unnecessary to continuously monitor lens element movement as said element is moved toward said known reference point. The cost savings attendant a unidirectional focus control system over a bidirectional focus control system makes it attractive to the control system designer. An example of a unidirectional focus control system for a variable focus camera lens is described in U.S. Pat. No. 3,713,371 to KURIHARA et al.
When utilizing a unidirectional focus control system for a variable focus lens, it is necessary to always return the movable element of such a lens to a known reference point as previously mentioned. As a practical matter, what this means is that an actuator must fairly rapidly drive said movable lens element into a fixed mechanical stop that functions as said known reference point, which often results in said movable element bouncing off and normally remaining at some fixed distance or at some offset from said fixed mechanical stop. The unidirectional focus control system will incorrectly assume that the movable lens element is resting against said mechanical stop or is positioned at said known reference point and may misfocus the lens in proportion to said bounce distance or offset. Even if the movable element is positioned precisely at a known reference point the mechanical coupling between said movable element and the actuator that is driving said element is normally fairly loose which will enable said actuator to move a finite distance before it moves said movable lens element, which may also result in a misfocusing of said variable focus lens by said unidirectional focus control system because of said finite distance movement.
In a variable focus control system, it is sometimes desirable to be able to utilize lens movement as the triggering means to initiate a lens related function or series of functions. If such an event is to be initiated well within the focusing range of said variable focus lens, lens movement can often by readily utilized as said triggering or initiating means. However, if the event is to be initiated precisely at either end of the variable focus lens focusing range, certain problems are encountered, especially if the event is to be both initiated and terminated at the same position. In such a situation, lens movement is required before an event is initiated at one end of the focusing range or the other. By definition then, the event cannot occur until the lens has moved, at least a finite distance away from its starting point, which necessarily causes an event initiation error that is proportional to said finite distance. This error can be minimized, but it cannot be entirely eliminated. Even when this error has been minimized to what might be deemed an acceptible level, it is very difficult, if not impossible, to construct switch means that will open and close at precisely the same point in response to lens element movement, whether such movement be rotational or translational. This difference between the opening and closing position of the movable element of a variable focus lens to open and close said switch means may introduce additional control system or event errors that might make the use of lens movement to initiate and/or terminate events at the focusing range end points of a variable focus lens, impractical.